The survey shows likely voters split 46% for Clinton and 44% for Trump -- with 5% supporting Libertarian Gary Johnson and 1% backing the Green Party's Jill Stein. That's well within the poll's 4.5 percentage point margin of error.

Among a broader group -- registered voters -- Clinton and Trump are tied at 41%, with Johnson at 7% and Stein at 2%.

The results show the race has tightened in recent weeks. In the same poll, Clinton led by 5 points in early September and by 8 points in early August.

A new CNN Poll of Polls incorporating the results of the new Post-ABC Poll finds the margin in an average of the five most recent polls holding at a three-point edge for Clinton, 43% back her, 40% Trump in that calculation.

There's been a similar tightening in the key swing state of Pennsylvania, a new Allentown Morning Call/Muhlenberg College survey shows. That poll, released Sunday, has Clinton at 40% to Trump's 38%, with Johnson at 8% and Stein at 3%. That's a drop for Clinton, who led by 8 points in the same poll a week earlier.

The ABC News/Washington Post poll also underscored that the 9 p.m. ET Monday debate at Hofstra University in New York will be a crucial moment in the campaign for both candidates. Eight in 10 voters said they plan to watch the debate -- with expectations slightly higher for Clinton, with 44% saying they expect her to win the debate while 34% expect Trump to win.